1. Filed of the Invention
The present invention relates to a method of preparing raw material powder for permanent magnets superior in moldability, especially in moldability and productivity with regard to bonded magnets.
2. Description of the Prior Art
Molded permanent magnets include sintered magnets and bonded magnets. Sintered magnets are prepared by sintering a raw material powder at a high temperature. Bonded magnets are prepared by binding raw material powder for magnets with such binders as rubbers and plastics. Bonded magnets are used widely, since the production process includes no sintering step, provides precision workpieces, eliminates machining like polishing, yields impact-resistant products and is suitable for mass-production of complexly molded products. As for the molding process, those used in plastics industries as rolling, extruding and injection are employed. The raw material powder preferably as much as possible has a spherical shape and a uniform particle size, in order to facilitate the molding process and improve the productivity. For example, in case of operating an injection molding machine, the more the powder for raw material of magnets approaches spherical shape and uniform particle size, the more the injection pressure decreases. Thus, it becomes possible to increase the productivity by increasing the rotation speed of the injection molding machine, and/or decrease the amount of molding auxiliary agents.
Raw materials for permanent magnets are developing remarkably, and Neodymium-Iron-Boron permanent magnets have being praised for the superior magnetic properties. JP-B-61-34242 discloses a magnetically anisotropic sintered magnet having a Fe . B . Nd components, and the production process includes providing a cast alloy of the above components and pulverizing mechanically the cast alloy to obtain a raw material powder.
However, the process has such drawbacks as requiring a pulverizing cost, and fluctuation in performance of products depending on production batches. The raw material powder has a broad range of particle size distribution due to the mechanical pulverization. The mechanically pulverized powder has little disadvantage as a raw material for sintered magnets. However, as a raw material for bonded magnets, the powder necessitates a higher injection pressure, and it is difficult to increase the productivity by increasing rotating speed of injection molding machines.
Further, a raw material powder for permanent magnets is proposed which is obtainable by reducing an acicular crystal of FeOOH (goethite) in a hydrogen gas stream at 300-600.degree. C. to turn to an acicular iron powder and dispersing in the iron powder such components for improving magnetic properties as a rare earth element like neodymium (Nd), boron and cobalt. However, since the starting raw material FeOOH (goethite) is an acicular crystal having an aspect ratio of from 5:1 to around 10:1, the obtained acicular iron powder has also an aspect ratio of larger than 5:1, which causes inferior moldability of the iron powder when used for production of bonded magnets.